Why do we kiss? A kiss is not just a kiss. It’s an activity teetering on the edge of gross when viewed objectively. Only those directly taking part in this exercise enjoy it. Kissing has been done throughout history. The earliest literate culture – the Sumer in Mesopotamia (today Iraq) – described standard and tongue kissing. The 1000 years old Kama Sutra from India devotes an entire chapter to various modes of creative lip-locking. Herodotus mentioned kissing among the Persians. During the Roman Empire everyone kissed everybody.

So why do we kiss? Women particularly have been shown to be sexually attracted to the scent of a man whose genes, coupled with her own, will create strong and beautiful offspring. So getting in close for a good whiff just makes genetic sense to her. Even within men the hormones the body releases during kissing create a sense of excitement, desire and stupidity. The first theory why we kiss is that it helps us assess whether a new mate is someone we want to have a long-term relationship with. While kissing we can smell and taste the partners pheromones, which carry information about how healthy the partner is.

The second reason we kiss is to help us maintain relationships, romantic ones or with family and friends. Our lips are full of nerve endings that are stimulated when you kiss someone, so you get a release of feel-good chemicals in the brain. The third reason why we kiss: we need it as an important part of the foreplay. Heavy kissing while having sex connects partners. And yes there is also the kissing within family members. Boring but socially necessary to keep the bonds in tact.

Whatever reason you have to kiss somebody, it helps transfer critical social and emotional information. But chemical signals play a role too in attraction. Scientists tested women – letting them sniff worn t-shirts of men and indicated which shirt smelled best to them. By comparing the DNA of the women and the men, researchers found that those ladies didn’t chose their favorite scent randomly. They preferred the scent of a man whose major histocompatibility complex MHC — a series of genes involved in our immune system — was different from their own. Having a different MHC means less immune overlap and a better chance of healthy, robust offspring. Kissing may be a subtle way for women to assess the immune compatibility of a mate, before she invests too much time and energy in him.